The present invention relates to the field of electrically tunable devices e.g. for microwave (radio frequency) circuits. Particularly it relates to a thin film ferroelectric varactor device, use of such a device in microwave (or millimeter wave) circuits and to a method of producing such a device.
Various tunable devices for use in microwave and millimeter wave devices have been proposed in the past. A varactor is a variable capacitance device in which the capacitance depends on a voltage applied thereto. Varactors are known to be used in RF tuning applications among others due to the fact that the capacitance variations of the varactor caused by an applied voltage has corresponding effects on frequency tuning.
Varactors based on semiconductors are known. However, such devices are disadvantageous in many aspects. They suffer e.g. from a low tunability (low dynamic range) at microwave frequencies, i.e. above 10-20 GHz, and the microwave losses are also high. Due to the inherent properties of semiconductor materials such varactors are susceptible to overheating and burnout if forward biased or reverse biased with an excessive applied voltage. Semicondutor PN junction devices have a depletion region which is subjected to high electric field stress, and as a consequence thereof, such devices may break down as the applied voltage is varied. Still further, semiconductor materials have dielectric constants between about 10-15, i.e. low dielectric constants which limits the capacitance and this is very disadvantageous for a plurality of applications.
Microelectromechanical varactors are also known. As opposed to semiconductor varactors they have a high dynamic range, or a high tunability and low microwave losses but the tuning speed is limited to tens of microseconds. In addition thereto they are sensitive to mechanical vibrations, they have a short lifetime and they are also not reliable.
Varactors based on ferroelectric materials or non-linear dielectrics are also known, e.g. from U.S. Pat. No. 5,472,935. The main disadvantage of the varactors disclosed in the above mentioned document, as well as other tunable microwave devices based on (bulk) ferroelectrics, is that the parameters are extremely temperature dependent which is related to the inherent temperature dependence of the ferroelectric materials. This is illustrated in FIG. 1A, FIG. 1B which show the extreme temperature dependence close to the maximum of the dielectric constant of typcial ferroelectric materials, Barium Titanate (BaTiO3, BTO) and Strontium Titanate (SrTiO3, STO). The dependence of the dielectric constant on an applied electrical DC field (the tunability) is also stronger close to the maximum of the dielectric constant whereas away from the maximum of the dielectric constant, the tunability is low. STO for example is not tunable at room temperature at a reasonably low applied electric field (E less than 100 kV/cm). This means that capacitors based on STO are not tunable at about room temperature (i.e. they are actually no varactors). This means that a high temperature stability only can be achieved in combination with a low tunability.
In addition thereto a transition layer is formed in the surface of the ferroelectric material at the interface between the electrodes of metal, e.g. gold, and the ferroelectric material in the varactor. The internal electric field of this layer will reduce the dielectric constant of the ferroelectric material and as a consequence thereof it also reduces the sensitivity to the applied external DC fields. In other words, the tunability of the varactor is reduced.
Ferroelectric varactors based on bulk material suffer among others from the drawback that the thickness of such devices limits the total capacitive effect.
It has been found advantageous to use thin ferroelectric films for the production of tunable capacitors since the dielectric constant of the ferroelectric films is tunable by variation of a voltage applied to the film. At high frequencies such films intrinsically show comparatively low losses.
U.S. Pat, No. 5,640,042 shows a simple ferroelectric varactor comprising a plurality of thin film layers. A carrier substrate layer is provided on which a metallic conductive layer is deposited. The thin film ferroelectric is in turn deposited on the metallic conductive layer and a plurality of longitudinally spaced metallic conductive means are disposed on the thin film ferroelectric. The carrier substrate layer, the metallic conductive layer and the thin film ferroelectric layer may have matching lattices to form a matched crystal structure. However, even if higher capacitance values than for example in semiconductor varactors can be obtained resulting in a higher tunability, such devices do not work satisfactorily for a plurality of implementations, e.g. because the temperature stability is not good enough and the extent to which such a device can be tuned is not sufficient.
What is needed is therefore an improved varactor device. More particularly a varactor device is needed which has a high dynamic range (a large range of tunability) and at the same time shows a high temperature independence, i.e. which shows a high degree of temperature stability. Particularly a ferroelectric varactor device is needed which is reliable, has a long lifetime and which do not suffer from mechanical stresses or vibrations or similar. Still further a varactor device is needed for which the tuning speed is high. The tuning speed can be defined as dC/dt, i.e. the time (t) derivative of the capacitance (C) and shows how fast the capacitance can be tuned. Further a ferroelectric varactor device is needed which is easy to fabricate and which moreover is not expensive to fabricate. Further yet a varactor device is needed which is suitable for a large number of applications, particularly for microwave or millimeter wave applications or even more particularly for microwave radio frequency applications. Particularly a varactor device is needed which has a high tunability (high dynamic range) and which is temperature independent in a given temperature interval.
A method of producing such a varactor device, fulfilling one or more of the above mentioned objects, is also needed which method particularly is easy to implement. A method of operating a tunable ferroelectric varactor device as referred to above is also needed.
Therefore a thin film ferroelectric varactor device comprising a substrate layer, a ferroelectric layer structure and an electrode structure is provided wherein the ferroelectric layer structure comprises a number of ferroelectric layers and a number of intermediate buffer layers arranged in an alternating manner. At least a first and a second of said ferroelectric layers, between which an intermediate buffer layer, which may be dielectric, is arranged, have different Curie temperatures. The Curie temperature is specifically defined as a temperature characterizing the temperature dependence of the dielectric constant. Specifically it is the temperature for which the dielectric constant has a maximum. According to the invention, different Curie temperatures for the respective ferroelectric layers is provided through giving ferroelectric layers a different chemical composition, or by chemically isolating the ferroelectric layers from one another, such that different Curie temperatures are provided. The content of at least one element of the elemental composition of the respective layer is different in the at least two layers. (Specially the content of an element may be zero in one of the layers.)
Advantageously at least some of the layers of the varactor device have lattice matched crystal structures. Even more particularly all layers, i.e. the layers of the ferroelectric layer structure, the electrode structure and the substrate layer, have lattice matched crystal structures.
In a preferred implementation the layers, particularly the intermediate buffer layers and the ferroelectric layers and the substrate layer comprise single crystalline films (epitaxial films).
In a preferred implementation the ferroelectric layers comprise a ceramic material. Particularly at least one element or component of the ceramic material is provided in a different fraction for a number of the layers or particularly for all layers, such that at least adjacent ferroelectric layers, wherein adjacent is taken to mean that ferroelectric layers between which an intermediate buffer layer, particularly a dielectric buffer layer, is provided, contain different fractions of said element(s).
In an advantageous implementation the ceramic materials comprise perovskite oxides or solid solutions thereof, ABCO3, wherein A for example is one of Ba, Na; B is anyone of e.g. Sr, Kr; C is is one of Tc, Nb etc. In a particular implementation the ferroelectric layers comprise BaxSr1-xTiO3. At least for the above mentioned first and second layers the Barium (Ba) content is different, thus for a first and a second layer the elemental composition is Bax1Sr1-x1TiO3 and Bax2Sr1-x2TiO3 respectively resulting in different Curie temperatures. Of course there may be more than two layers having different xi values or specifically different Barium-content. Particularly is for each ferroelectric layer i, 0xe2x89xa6xixe2x89xa61.
In an alternative implementation the at least two layers comprise NaxiK1-xiNbO3 wherein xi is different for, at least two xe2x80x9cadjacentxe2x80x9d ferroelectric layers. In a particularly advantageous implementation the intermediate buffer layers comprise dielectric films. The dielectric films between different ferroelectric layers may have the same elemental composition or different elemental compositions.
In a particular implementation, at least one of the dielectric films has an elemental composition of MgO, LaAlO3, CeO2 or a material with similar properties. If the ferroelectric structure is such that there are more than one intermediate buffer layer, e.g. if there are more than two ferroelectric layers requiring an intermediate buffer layer between them, or, if an intermediate layer also is provided between a ferroelectric layer and the substrate layer, each dielectric buffer layer may be of the same elemental composition but it does not have to. In a particularly advantageous implementation at least one of the dielectric intermediate buffer layers has an elemental composition of WO3.
According to different implementations one dielectric layer has an elemental composition of WO3 whereas one or more other layers comprise MgO or similar; alternatively all of the layers comprise WO3. In still another implementation, at least one of the intermediate buffer layers comprises a multilayer structure comprising a number of sublayers, with at least one sublayer with an elemental composition of MgO or similar, as referred to above, and at least one sublayer with an elemental composition of WO3. The substrate layer may have an elemental composition of MgO, LaAlO3 or a material with similar properties whereas the electrode structure may comprise longitudinally arranged electrodes defining a gap between them. The electrodes may for example comprise gold (Au), copper (Cu), silver (Ag) or similar but they may also comprise superconductors or particularly high temperature superconductors, of YBCO (YBaCuO) or TBCCO (TlBaCaCuO).
In a preferred implementation the ferroelectric layers have a thickness smaller than, or equal to, 1 xcexcm. The intermediate buffer layers, which particularly are dielectric, may have a thickness of 100 nm or less. They may be also be somewhat thicker in case they consist of a combination of WO3 and MgO (also than they may however be as thin as referred to above) or similar, or if they comprise a multilayer structure. In another implementation, the ferroelectric layer structure comprises a ferroelectric nanostructure with ultra thin ferroelectric layers having a thickness substantially smaller than or equal to 100 nm. A buffer layer may be provided adjacent to the electrode structure, i.e. between the electrodes and a ferroelectric layer or an intermediate buffer layer (a dielectric layer). Such buffer layer may for example consist of thin metallic Mg films.
The ferroelectric device may particularly comprise a ferroelectric layer structure comprising three or more ferroelectric layers wherein between each pair of ferroelectric layers a preferably dielectric, intermediate buffer layer is provided. An intermediate buffer layer may also be provided between the substrate layer and the ferroelectric layer to be deposited on the substrate layer.
Particularly the temperature dependence of the capacitance of the varactor can be controlled by selection of the Curie temperatures/the xi values in the elemental compositions of the respective ferroelectric layers, i.e. the content of at least one element of the elemental composition.
A thin film ferroelectric varactor device comprising a substrate layer, a ferroelectric layer structure and an electrode structure is provided in which the ferroelectric layer structure comprises a number of ferroelectric layers and a number of intermediate buffer layers, which preferably are dielectric, wherein the ferroelectric layers and the dielectric layers are arranged in an alternating manner such that ferroelectric layers between which an intermediate buffer layer is provided, are chemically separated from each other. At least some of the ferroelectric layers have a different elemental composition. Particularly the ferroelectric layers comprise ceramic materials, such as perovskite oxides or solid solutions of the type AxiB1-xiCO3 wherein xi is different for at least some subsequent ferroelectric layers between which an intermediate buffer layer is provided. Particularly, for each layer i, 0xe2x89xa6xixe2x89xa61. Particularly the ferroelectric layers i, wherein i, . . . , N; N being the number of ferroelectric layers of the structure, comprise Baxi Sri-xi TiO3 or Naxi K1-xi NbO3. Particularly all layers of the varactor device have matching crystal structures and the Curie temperatures of the respective ferroelectric layers are different, wherein the respective Curie temperatures are given by the selection of each xi, and the selection is done so as to assure that the maximum values of the dielectric constants of the respective layers are different. Particularly the temperature dependence of the capacitance of the varactor can be controlled. Particularly the Ba-content/the Na-content is different for each respective layer (having as a consequence that also the Sr/K content is different).
According to the invention the varactor devices as discussed above may with advantage be used in microwave (radio frequency) circuits, such as tunable resonators, filters, phase shifters, delay lines, mixers, harmonic generators or similar.
A method of producing a thin film ferroelectric varactor device comprising a substrate layer structure, a ferroelectric layer structure and an electrode structure is also given which includes the steps of; providing a ferroelectric layer structure on the substrate layer structure, including the steps of; providing an intermediate, preferably dielectric, buffer layer between each of a number of ferroelectric layers; for at least two ferroelectric layers (i;i+1), selecting different contents (xi,x(i+1)) of a first component of the elemental compositions of the layers; selecting the contents (xi,x(i+1)) such that the dielectric constants of different layers will have different Curie temperatures. The varactor may then be operated between the Curie emperatures of the two ferroelectric layers.